1. Field of the Invention
The present invention relates to apparatus and methods for the treatment of presbyopia using ultraviolet and infrared lasers to ablate the sclera tissue of an eye.
2. Prior Art
Corneal reshaping, including a procedure called photorefractive keratectomy (PRK) and a new procedure called laser assisted in situ keratomileusis, or laser intrastroma keratomileusis (LASIK), has been performed by lasers in the ultraviolet (UV) wavelength of 193-213 nm. Commercial UV refractive lasers include ArF excimer lasers at 193 nm and other non-excimer, solid-state lasers, such as the one patented by the present inventor in 1992 (U.S. Pat. No. 5,144,630). Precise, stable corneal reshaping requires lasers with strong tissue absorption (or minimum penetration depth) such that the thermal damage zone is at a minimum (less than few microns). Furthermore, accuracy of the procedure of vision correction depends on the amount of tissue removed in each laser pulse, in the order of about 0.2 microns. Therefore, lasers at UV wavelengths between 193 and 213 nm and at the mid-infrared wavelengths between 2.8 and 3.2 microns are two attractive wavelength ranges which match the absorption peak of protein and water, respectively.
The above-described prior arts are however limited to the use of reshaping the corneal surface curvature for the correction of myopia, astigmatism and hyperopia. When a person reaches a certain age (around 45), the eyes start to lose their capability to focus for near vision and become presbyopic. Presbyopia is not due to the cornea curvature but comes about as the lens loses its ability to accommodate or focus for near vision as a result of loss of elasticity that is inevitable as people age. Therefore the existing lasers using corneal reshaping can not provide the solution for presbyopia patients. In addition, corneal reshaping is ablating the central portion of the cornea and changing its curvature.
To correct presbyopia, the present patent uses a “cold” laser to remove sclera tissue (outside the limbus area) versus a “thermal” lasers in Sand's patent (U.S. Pat. No. 5,484,432) to shrink the corneal shape (inside the limbus area). The cold laser of the present has a wavelength range of (0.15-0.36) microns and (2.6-3.2) microns which are also different from that of the “thermal” laser range of (1.80-2.55) microns proposed by Sand.
The prior arts of Ruitz (U.S. Pat. No. 5,533,997) and Lin (U.S. Pat. No. 5,520,679) are all limited to the corneal central portion and are designed to change the curvature of the cornea by ablation of the surface layer of the cornea. The present system, on the contrary, does not change the corneal central curvature and only ablates tissue outside the limbus.
The technique used in the prior art of Bille (U.S. Pat. No. 4,907,586) is specified to below conditions: (a) quasi-continuous laser having pulse duration less than 10 picoseconds and focused spot less than 10 micron diameter; (b) the laser is confined to the interior of a selected tissue to correct myopia, hyperopia or astigmatism, and (c) the laser is focused into the lens of an eye to prevent presbyopia. He also proposed to use laser to create a cavity within the corneal stroma to change its visco-elastic properties.
The “presbyopia” correction proposed by Ruitz using an excimer (ArF) laser also required the comeal surface to be reshaped to form a “multifocal” effect for a presbyopia patients to see near and far. However, Ruitz's “presbyopia” correction is fundamentally different from that of the present system which does not change the corneal curvature and only ablate the scleral tissue outside the limbus area. In the present system, we propose that the presbyopia patient is treated by increasing the patient's accommodation rather than reshaping the cornea into a “multifocal” configuration.
To treat presbyopic patients, or the reversal of presbyopia, using the concept of expanding the sclera by mechanical devices or implantation of a band has been proposed by Schachar in U.S. Pat. Nos. 5,489,299, 5,722,952, 5,465,737 and 5,354,331. These mechanical approaches have the drawbacks of complexity and are time consuming, costly and have potential side effects. To treat presbyopia, the Schachar U.S. Pat. Nos. 5,529,076 and 5,722,952 propose the use of heat or radiation on the corneal epithelium to arrest the growth of the crystalline lens and also propose the use of lasers to ablate portions of the thickness of the sclera. However, these prior arts do not present any details or practical methods or laser parameters for the presbyopic corrections. No clinical studies have been practiced to show the effectiveness of the proposed concepts. The concepts proposed in the Schachar patents regarding lasers suitable for ablating the sclera tissues were incorrect because many of his proposed lasers are thermal lasers which will cause thermal burning of the cornea, rather than tissue ablation. Furthermore, the clinical issues, such as locations, patterns and depth of the sclera tissue removal were not indicated in these prior patents. In addition, Schachar's methods also require the weakening of the sclera and increase the lens diameter by band expansion, which is different from the theory proposed in the present patent, where the sclera tissue becomes more flexible than weakening after laser ablation.
Another prior art proposed by Spencer Thornton, Chapter 4, “Survey for hyperopia and presbyopia”, edited by Neal Sher (Williams & Wilkins, MD, 1997) is to use a diamond knife to incise radial cuts around the limbus areas. It requires a deep (90%-98%) cut of the sclera tissue in order to obtain accommodation of the lens. This method, however, involves a lot of bleeding and is difficult to control the depth of the cut which requires extensive surgeon's skill. Another major drawback for presbyopia correction provided by the above-described non-laser methods is the post-operative regression of about (30%-80%) caused by the healing of the “incision” gap. And this regression is minimum in the laser “excision” or “ablation” method proposed in the present invention.
The important concept proposed in the present invention is to support the present inventor's post-operative results which show minimum regression. We proposed a theory based upon the fact that the laser ablated sclera tissue “gap” will be filled in by the sub-conjunctiva tissue within few days after the surgery. This filled in sub-conjunctiva tissue is much more flexible than the original sclera tissue. Therefore the filled-in gap in the sclera area will cause the underlying ciliary body to have more space to move. This in turn, will allow the ciliary body to contract or expand the zonular fiber which is connected to the lens, when the presbyopic patient is adjusting his lens curvature to see near and far. The above described sub-conjunctiva tissue filling effects and the increase of “flexibility” of the sclera area are fundamentally different from the scleral “expansion” (or weakening) concept proposed by the prior arts of Schachar who proposed an implanted sclera band. In the present invention, the laser ablated sclera area is not weakening, it becomes more flexible instead.
Therefore one objective of the present invention is to provide an apparatus and method to obviate these drawbacks in the above described prior arts.
It is yet another objective of the present system to use a scanning device such that the degree of ciliary muscle accommodation can be controlled by the location, size and shape of the removed sclera tissue.
It is yet another objective of the present invention to define the non-thermal lasers for efficient tissue ablation.
It Is yet another objective of the present system to define the optimal laser parameters and the ablation patterns for the best clinical outcome for presbyopia patients, where scleral ablation will increase the accommodation of the ciliary muscle by the increase of the flexibility in the laser-ablated areas.
It is yet another objective of the present system to provide the appropriate scanning patterns which will cause effective ciliary body contraction and expansion of the zonules and the comeal lens based upon a theory different from the prior art. It is yet another objective of the present system to provide a new mechanism which supports the clinical results of laser presbyopia correction with minimum regression. One important concept proposed in the present system is to support the post-operative results which show minimum regression when presbyopia is corrected by a laser ablation of the sclera tissue. We proposed that the laser ablated sclera tissue “gap” is filled in by the sub-conjunctiva tissue within a few days after the surgery. This filled-in sub-conjunctiva tissue is much more flexible than the original sclera tissue. Therefore the flexible filled-in gap in the sclera area will allow the ciliary body to contract and cause the zonular fiber and the corneal lens to adjust its focusing power and increase the accommodation of presbyopic patient.
The concept presented in the present patent is to remove, by any methods including laser or non-laser methods, portion of the sclera tissue which is then filled in by sub-conjunctiva tissue to increase the flexibility of the scleral area and in turn causes the movement of the ciliary body and zonular fiber to increase the lens accommodation.